A plug used in a piercing mill pierces a heated billet (round billet) to form a hollow pipe or tube. As shown in FIG. 11, a plug 100 is provided between a pair of inclined rolls 150 each inclined with respect to a pass line PL. The plug 100 is pressed into a billet 50 rotated by the inclined rolls 150 in the circumferential direction, and then the piercing mill pierces and rolls the billet 50 along its central axis to form the billet into a hollow pipe or tube 51.
The plug 100 is in contact with the billet 50 in the piercing and rolling process and subjected to heat and strong pressure from the billet 50, and therefore its surface is likely to be eroded. As disclosed by JP 9-29310 A, the eroded plug is re-grinded in the axial direction for reuse. More specifically, as shown in FIG. 12, when the plug 100 has erosion 110 at a surface SF1, the plug 100 is re-grinded in the axial direction until the erosion 110 at the surface is removed. At the time, a plug surface SF2 after the re-grinding has the same shape as that of the original surface SF1. In this way, the plug can be reused because the surface SF2 of the plug has substantially the same shape as that of the original surface though the overall plug length is reduced. The overall length of the plug is however reduced every time the plug is re-grinded, and therefore the number of re-grinding is limited. Therefore, although the plug can be reused, the plug has a shorter useful life if it is frequently eroded.
A plug having a shape that allows erosion to be reduced is disclosed by JP 57-50233 A and WO 2004/052569 pamphlet. As shown in FIG. 13, the plug 200 disclosed by the documents includes a semi-spherical tip end portion 201, a columnar portion 202, and a middle portion 203 sequentially from the front end. When a billet 50 is pierced using the plug 200, a gap IS is formed between the billet 50 pierced by the tip end portion 201 and the surface of the columnar portion 202. In this way, the columnar portion 202 is not in contact with the billet 50, less heat is transmitted from the billet to the plug 200, and the gap IS allows heat stored in the plug 200 to be dissipated. Therefore, the plug 200 is less likely to be eroded as compared to a plug 100 having a conventional shape.
The plug 200 is however not suited for reuse by re-grinding. As shown in FIG. 14, if erosion 210 about as deep as the erosion 110 in FIG. 12 is generated at the columnar portion 202, a re-grinding allowance Lc necessary for returning the columnar portion 202 into the original shape is excessively greater than that of the plug 100. This is because the outer diameter of the columnar portion 202 is fixed, and the plug must be re-grinded for a length almost equal to the length of the erosion 210 to remove the erosion 210, or the columnar portion 202 cannot be returned to the original shape. Therefore, the overall length of the plug 200 after such re-grinding is too short for reuse.
In order to reduce the re-grinding allowance Lc for the plug 200, the outer diameter of the columnar portion 202 may be increased gradually from the front end side to the back end side of the plug so that the portion has a tapered shape. However, in the tapered shape, the gap IS is not formed between the portion and the billet in the piercing process, so that the billet and the tapered portion contact with each other and erosion is more easily caused.